Image-forming MR methods, which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, they do not require ionizing radiation, and they are usually not invasive. MRI is used for example as imaging technique to visualize myocardial injury. Cardiac and respiratory triggered MR imaging can be used to image morphology, time resolved cine movies may reveal cardiac function, dynamic contrast enhanced imaging can be utilized to measure perfusion and MR tagging sequences can be used to study the contraction of the myocardium in detail.
According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field BO whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based.
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.
In order to realize spatial resolution in the body, switching magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body.
The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of samples of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
A major criterion for obtaining high quality MR images is to ensure that the imaged region of interest is not moving during an MR scan. In case of for example abdominal imaging this becomes a serious problem since physically necessary patient breathing and thus patient movement translates into blurring and ghosting of the acquired MR image. Consequently, a breath hold is required by the patient during the MR imaging scan in order to prevent any movement in the imaged region of interest.
U.S. Pat. No. 7,182,083 B2 discloses an integrated respiratory monitor and CT imaging device apparatus. The respiratory monitor system is adapted to engage a patient and generate a respiratory signal representative of a breath hold level of the patient during a breath hold. The imaging device is adapted to scan the patient during the breath hold and generate a volumetric image data set of the patient. The respiratory sensor and imaging device are operatively connected to associate the respiratory signal representative of the breath hold level of the patient together with the volumetric image data set of the patient.
Applying breath hold commands and starting an imaging scan when the patient has reached the breath hold state can introduce operator dependent variations. Breath hold sequences typically are started too early when the breath hold state has not been reached yet which translates into blurring and ghosting of the image. In the breath hold state, the breath hold may “drift away” also leading to motion related problems. Also automated breath hold commands are not patient dependent thus neglecting the capability to follow the breath hold commands and to address the overall patient situation. The patient's breath hold capabilities typically are related to the progress and the severity of a patient's disease.